Researchers have reported that holographic memory systems are capable of providing relatively fast retrieval of large amounts of data compared to conventional magnetic disk storage systems or compact disk-read only memories (CD-ROM's). Data is typically stored in a holographic memory as a hologram formed in a photo-sensitive storage medium. The hologram represents data as a pattern of varying indexes of refraction, absorption, polarization and/or reflectivity in the storage medium.
The hologram pattern is typically formed by causing two particular light beams to intersect at a region of the storage medium. These two light beams are referred to as the signal beam and the reference beam. The signal beam is often formed by transmitting laser light through a spatial light modulator (SLM), such as a liquid crystal display (aLCD) screen. The LCD screen contains a pattern of clear and opaque regions corresponding to a two-dimensional depiction of the digital data to be stored. The laser light signal emanating from the LCD screen is then relayed by lenses to create the signal beam.
In systems using Bragg selectivity as a means for recording the hologram, the reference beam is a collimated light beam having particular characteristics, such as a particular wavelength and angle of incidence with respect to the storage medium. In such systems, the recorded data can be read out or played back by illuminating the storage medium with a collimated light beam having the same characteristics as the reference beam used for recording the hologram pattern. The storage medium defracts, absorbs and/or polarizes the playback reference beam in such a manner to cause a projected image of the original data representation on a photodetector array, such as a charged-coupled device (CCD).
A typical CCD array contains an array of sensing elements which operate by simultaneously sensing light and dark patterns of the projected data image. Then, although the amount of light is detected simultaneously by each sensor element of the CCD array, electrical signals representing such detected light can only be sequentially read out by rows or columns. These electrical signals can then be used by a processing system such as a computer system. Digital holographic memories are described generally in D. Psaltis and F. Mok, "Holographic Memories", Scientific American, pp. 70-76 (November, 1995), which is incorporated by reference herein.
Relatively large information densities have been achieved with holographic memory systems by recording successive pages of data onto common regions of a holographic storage medium. Such recording of different pages can be achieved using Bragg selectivity for differentiating between successively recorded pages, such as by varying the angle of incidence or wavelength of the reference beam.
Nevertheless, commercial applications of holographic memory systems for storing and retrieving data are currently not available. In the read out of a recorded data page from the storage medium, the particular system characteristic employed for storing the data page must be substantially precisely duplicated to project the data page image on the photodetector array. In systems employing Bragg selectivity for recording multiple data pages, the required precision of the reference beam for read out is dependent on the thickness of the storage medium. Thicker mediums require greater precision. For instance, if the storage medium is 1 cm thick and the reference beam angle deviates by 0.001.degree. or approximately 2.times.10.sub.31 5 rad, then the projected data page can effectively disappear.
During playback, thermal expansion and other thermal effects as well as physical and optical disturbances of the mechanical relationship between the holographic system components can cause variations of the system characteristics from those originally used for recording. Such variations tend to interfere with distinguishing the reading of one data page from another. Known systems do not provide effective compensation for such variations. Moreover, such variations or differences are particularly troublesome with movement of the storage medium from one holographic system to another such as from a recording system to a playback system.
Further, reported systems do not provide a technique to compensate for misalignment between photodetector array sensor elements and the projected pixel images of a data page image. Without such alignment or pixel registration, projected pixels of the data page may overlap adjacent sensor elements. Such overlap makes reading of the data very difficult.
As a result, a need exists for a technique that provides rapid and efficient access to and read out of a data page in a holographic system.